1. Institute for Materials Research,Tohoku University,Ibaraki 311-1313,Japan
2. Japan Society for the Promotion of Science,Tokyo 102-0083,Japan
3. Graduate School of Science,Tohoku University,Sendai 980-8579,Japan
4. Faculty of Engineering,Okayama University of Science,Okayama 700-0005,Japan
5. Tokyo Tungsten Co.Ltd.,Toyama 931-8543,Japan